EFFECTS OF YTTRIA AND ZIRCONIA ADDITION ON MICROSTRUCTURE AND CYCLIC OXIDATION RESISTANCE OF ODS STEEL CONSOLIDATED BY SPARK PLASMA SINTERING METHOD

• Main Author: AZMI RABBANI, NAUFAL
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/29573
• Institution: Institut Teknologi Bandung

The development of nuclear power generation technology has entered Generation IV. Materials having high resistance of high temperature oxidation and neutron irradiation become the focus of research for development. In supercritical water reactor (SCWR), especially in the cladding section of pressure vessel, ODS ferritic steel becomes one of the most suitable material candidates to be applied under these operating conditions. In addition to being able to operate under high temperature oxidation conditions and high neutron irradiation, this type of steel also has resistance to thermal embrittlement and fatigue crack growth, and has a high mechanical strength. Further studies have shown that ferritic ODS steels with a composition of 16 - 17 wt.% Cr, 3.5 – 4 wt.% Al have the best oxidation resistance. <br /> <br /> In this experiment, the basic composition of ferritic steel ODS Fe17Cr3.5AlTiY ('FP') was combined with 3 different compositions, ie 0.4% Y2O3 ('FY'), 0.4% ZrO2 ('FZ' ), and 0.2% Y2O3 + 0.2% ZrO2 ('FYZ') addition. The four samples (FP, FY, FZ, FYZ) were then milled using a planetary ball mill with hexan as a milling solution for 6 hours to allow mechanical alloying and size reduction. The mechanical alloyed samples were sintered by spark plasma sintering (SPS) method at 1000oC with a heating rate of 100oC / min without holding time. The four sintered samples were conditioned by polishing before tested for density, porosity, hardness, cyclic oxidation, XRD, and SEM. The cyclic oxidation test was carried out at a temperature of 800oC for 8 cycles. One cycle of oxidation test consisted of 20 hours of heating and 4 hours of cooling. Density test was done using Archimedes method. Porosity test was performed using 200x and 500x magnification photographs of optical microscopy and ImageJ as a software for the analysis. Hardness test was performed using Vicker Hardness Test. XRD test for phase and microstrain analysis was performed on the sample after milling, sintering, and oxidation test. SEM-EDS test for microstructure analysis and elemental distribution was performed on samples after sintering and oxidation testing. <br /> <br /> The test results showed that alloy density from highest to lowest and porosity of alloys from lowest to highest were FY, FP, FYZ, FZ. Hardness from highest to lowest was FY, FYZ, FZ, FP. The cyclic oxidation resistance from highest to lowest was FZ, FYZ, FP, FY. The phases formed in the sample after mechanical alloying and after sintering were a-Fe, while in all samples after oxidation test were a-Fe, a-Al2O3, and FeTiO3, with Cr2O3 as additional phase present only in FY sample. The comparison between the results of data analysis and literature study showed that high porosity in samples with ZrO2 addition was found to be caused by the change of crystal structure from monoclinic to tetragonal causing internal cracking. This condition occurred due to the presence of ZrO2 which has not been stabilized to cubic structure even after the addition of Y2O3. The higher density and lower porosity in the sample without the addition of ZrO2 became the main cause of higher hardness in FY. In FP, the hardness value was low due to the absence of dispersant oxide in the composition. The oxidation resistance of the sample was affected by the oxide / metal interface resistance to crack propagation, as well as the presence of residual stress. By adding ZrO2 and limiting the addition of Y2O3 into the alloy, the occurrence of crack propagation on the oxide/metal interface causing the discontinuity of the oxide layer could be prevented.